Update STRING in kg-registry

[caufieldjh]

Description

Add STRING (protein-protein interaction network) to the kg-registry.

Resource Details

• Domain: Protein–protein associations
• Paper: Yes — STRING papers in peer-reviewed journals
• Website: Yes — https://string-db.org/
• API: Yes — REST API / programmatic access
• Downloadable graph data: Yes — bulk downloads available

Parent Issue

Relates to #48 - Survey environmental KGs and add missing ones

Additional Notes

Comprehensive database of protein-protein interactions with confidence scores and multiple evidence types.

[caufieldjh]

STRING is already in the registry, but needs some improvement:

• Description
• Verify current Products and versions